SSINS No.: 6835
Accession No.:
8005050068
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
June 11, 1980
Information Notice No. 80-27
DEGRADATION OF REACTOR COOLANT PUMP STUDS
Description of Circumstances:
On May 17, 1980, the NRC staff was informed by Omaha Public Power District
(OPPD) that severe corrosion damage was found on a number of closure studs
in two of the four Byron Jackson reactor coolant pumps at Fort Calhoun Unit
1 (PWR).
At the time, the reactor coolant system was undergoing a routine low
pressure leak test (180 psig) and visual inspection prior to plant restart
after a four-month outage for refueling, pipe support modifications and
scheduled inservice inspection. During the visual inspection, saturated and
dripping insulation was observed at one of the Byron Jackson reactor coolant
pump flange regions. Upon removal of the insulation, evidence of coolant
leakage was found emanating from the seating surfaces between the pump
casing and the pump cover. Further investigation of the three remaining
pumps indicated similar coolant leakage past both inner and outer flange
gaskets on two of the three pumps. After complete removal of the nonmetallic
insulation, further visual observations revealed three studs located
side-by-side on one pump and three studs similarly located on the other pump
had significant corrosion wastage in the shank area next to the lower thread
section in the pump casing flange. Wastage of approximately 50% of the
original diameter of the stud giving them an "hour-glass" type appearance
was observed. The corroded studs were located in the vicinity of a component
cooling water line on both affected pumps but no direct correlation of this
fact has been established. Although not confirmed by metallurgical analysis,
the cause of the stud wastage is thought to be corrosive attack by hot boric
acid from the primary coolant.
The pump cover and casing for these pumps are constructed of ASTM A-351,
Grade CF8M stainless steel. Sealing between the cover and casing is achieved
by two concentric 304 stainless steel flexitallic gaskets. A leak-off line
installed between the gaskets on each pump was plugged and not instrumented.
The leak-off line was not in use and therefore, no indication of RCS leakage
from the inner seal was available. Each pump has 16 closure studs,
consisting of ASTM A-193 Grade B7 low alloy steel, which are chrome plated
in the thread area and phosphate coated in the shank area. The studs are
approximately 3-1/2 inches in diameter and about 29 inches long.
No maintenance requiring removal of the pump casing studs had been performed
on the reactor coolant pumps since initial construction. The studs were
covered
.
Information Notice No. 80-27 June 11, 1980
Page 2 of 2
with block type insulation since construction thereby limiting them from
view. The studs on two pumps were ultrasonically inspected in place in
accordance with the applicable ASME Section XI code rules. These ultrasonic
examinations were intended to locate cracks in bolting and were not
effective in revealing wastage of the studs.
The three affected pumps will be disassembled for further cleaning and
inspection of the studs and mating surfaces. Prior to reassembly, all studs
exhibiting significant corrosion will be replaced. All new, or acceptable
used studs, will be subjected to ultrasonic, visual and magnetic particle
examinations. Installation of instrumentation for actively monitoring the
leak-off lines between the flexitallic gaskets is being performed. Future
inservice inspections, presently limited to ultrasonic examination, will be
supplemented with visual examination of the studs installed in the reactor
coolant pumps. Replacement insulation will be in the form of a removable
blanket to facilitate visual examination.
The condition of the studs discovered at Ft. Calhoun raises concerns that
such severe corrosion, if undetected, could lead to stud failures which
could result in loss of integrity of the reactor coolant pressure boundary.
The lack of effectiveness of current ultrasonic examinations in revealing
wastage emphasizes the need for supplemental visual inspections and use of
instrumented leak detection systems to preclude unacceptable stud
degradation going undetected. Licensees should consider that the potential
for undetected wastage of carbon steel bolting by a similar mechanism could
exist in other components such as valves.
This Information Notice No. is provided as an early notification of a
significant matter that is still under review by the NRC staff. It is
expected that recipients will review the information for possible
applicability to their facilities.
No specific action or written response to this Information Notice No. is
required. If NRC evaluations so indicate, further licensee actions may be
required.